Field evaluation of an ELISA to detect antibody in leprosy patients and their contacts.

Previous studies have detected circulating antibody in leprosy using a variety of difficult laboratory methods. We have developed a simpler method for detecting antibody by ELISA, using autoclaved Mycobacterium smegmatis as the antigen. Evaluation was performed on eluates from 25 microliter aliquots of finger-prick blood dried on filter-paper disks in two high-incidence populations in Ponape, Micronesia. Among 228 nonleprosy cases bled in 1980 and rebled and re-examined in 1982: a) for those who had been ELISA positive two years earlier, the leprosy attack rate during the intervening two years was at least twice as high as among those who had been negative, and we estimate that shortening the screening interval to one year plus doing confirmatory retests on new sero-converters would increase the relative risk (or "predictive power") to over sixfold, including all impending multibacillary cases; b) elevated antibody levels were detected up to two years prior to clinical onset of disease in 70% of new cases; and c) both asymptomatic conversion (rising titer) and reversion (falling titer) were observed. Among 150 biopsy-proven cases, ELISA results suggest that fall of titer in most uncomplicated paucibacillary cases was rapid (months), but in multibacillary cases was more gradual (years), probably paralleling responses to treatment with titers rising in reactivation. These results suggest that this technique, with an improved antigen, may be useful in leprosy control programs, both for detecting candidates for preventive treatment and for following responses to therapy.